Nanostructured titanium oxide material and its synthesis procedure

ABSTRACT

Nanomaterials of the JT phase of the titanium oxide TiO 2-x , where 0≦x≦1 having as a building block a crystalline structure with an orthorhombic symmetry and described by at least one of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb. The nanomaterials are in the form of nanofibers, nanowires, nanorods, nanoscrolls and/or nanotubes and are obtained from a hydrogen titanate and/or a mixed sodium and hydrogen titanate precursor compound that is isostructural to the JT crystalline structure. The titanates are the hydrogenated, the protonated, the hydrated and/or the alkalinized phases of the JT crystalline phase that are obtained from titanium compounds such as titanium oxide with an anatase crystalline structure, amorphous titanium oxide, and titanium oxide with a rutile crystalline structure, and/or directly from the rutile mineral and/or from ilmenite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 12/848,392,filed Aug. 2, 2010, which, in turn, is a divisional application of Ser.No. 12/623,993, filed Nov. 23, 2009, now U.S. Pat. No. 7,799,313.Application Ser. No. 12/623,993 is a divisional application of Ser. No.11/121,178, filed May 4, 2005, now U.S. Pat. No. 7,645,439. ApplicationSer. No. 11/121,178 is a continuation-in-part of InternationalApplication No. PCT/MX2004/000035, with an international filing date ofMay 26, 2004, and International Application No. PCT/MX2003/000081, withan international filing date of Oct. 10, 2003. This application alsoclaims the benefit under 35 U.S.C. §119(a) of Mexican Patent ApplicationNo. PA/a/2004/004265, filed May 4, 2004, and under 35 U.S.C. §119(e) ofprovisional application Ser. No. 60/646,973, filed Jan. 27, 2005. Thedisclosures of all of the foregoing applications are incorporated hereinby reference in their entirety.

Reference is made to application Ser. No. 11/121,179, filed May 4, 2005,of Jose Antonio Toledo Antonio et al., entitled “Selective AdsorbentMaterial And Its Use”, now U.S. Pat. No. 7,416,655, which issued on Aug.26, 2008.

FIELD OF THE INVENTION

The present invention relates to nanomaterials of titanium oxide(TiO_(2-x), where 0≦x≦1), which have as a building block a crystallinestructure with an orthorhombic symmetry. The new crystalline structureis the basic unit of construction of nanomaterials which are nanofibers,nanowires, nanorods, nanoscrolls and/or nanotubes. These nanomaterialsare obtained from a precursor that is isostructural to the newcrystalline structure. The precursor is composed of hydrogen titanateand/or a mixed sodium and hydrogen titanate. These titanates are thehydrogenated, protonated, hydrated and/or the alkalinized phases of thenew crystalline structure. In addition, this invention also relates tothe procedure of synthesis of the nanomaterials.

BACKGROUND OF THE INVENTION

Researchers in the catalysis and materials fields have focusedconsiderable efforts on the design of new porous materials, eithersynthetic or natural with enhanced textural properties, throughinnovating synthesis procedures as the molecular molding. Generally theporous structure of such solids is formed during its crystallization orduring further treatments.

The porous materials are classified, depending on their predominatingpore size, as: 1) microporous, with pore sizes <1.0 nm; 2) mesoporous,with pore sizes between 1.0 and 50.0 nm, and 3) macroporous, with poresizes surpassing 50.0 nm. Of all of them, the macroporous solids have alimited use as adsorbers or catalysts due to the fact that theygenerally present a low surface area and their large pores are notuniform. On the other hand, the microporous and mesoporous solids arewidely used in the technologies of adsorption, separation and catalysis,particularly for the processing and refining of oil. For suchapplications, nowadays, there is an increase in the demand of newmaterials with a well defined and homogeneous pore distribution,thermally stable, with a high specific area and large pore volumes; inorder to make more efficient the physical and/or chemical processes inwhich these materials are used.

The porous materials can have an amorphous or nanocrystalline structure.The amorphous materials, such as silica gel or alumina gel, do not haveany crystallographic order, while nanocrystalline solids such astransition alumina, gamma or eta, present a partially orderedstructured. Generally, these two kinds of materials display a very widepore distribution, which limits their effectiveness as catalysts,adsorbents and/or ionic exchange systems. The wide pore distributionlimits mainly the use of these materials in oil refining processes.

Zeolites and the molecular sieves are a clear example of uniformity inthe pore sizes that have to be rigorously established. However, the poresize distribution is limited to the microporous region, due to the factthat the pores are formed from the cavities and/or channels that formthe structure itself; therefore, molecules of big dimensions cannot beprocessed in this type of materials. On the other hand, these materialsare generally synthesized under hydrothermal conditions in the presenceof a porogen agent that engineers the porous structure.

The need to expand the uniformity and the homogeneity of the pore sizesfrom the microporous region to the mesoporous region, thus allowing theadsorption and processing of bigger molecules, has led to the search ofnew organic agents capable of engineering new structures. This has givenorigin to molecular sieves with bigger pore size as thealuminophosphates, galliophosphates, etc. (Nature, vol. 352, 320-323(1991); J. Chem. Soc. Chem. Commun., 875-876 (1991)). However, thesestructures are not thermally stable.

With the discovery of the mesoporous silicates and aluminosilicates in1992 (U.S. Pat. Nos. 5,098,684 and 5,102,643), a new stage in thedevelopment of ordered mesoporous materials started. This type ofmaterials, called M41S, have a uniform pore size, which is adjustable toan interval between 1.3 and 10.0 nm. Such materials display a pore wallwith a thickness between 0.8 and 1.2 nm and a crystal size over 50.0 nm.On the other hand, depending on the general conditions of synthesis, inparticular on the concentration of the organic porogen agent, the M41Smaterials can have an hexagonal morphology (MCM-41), a cubic morphology(MCM-48), or a laminar structure (J. Am, Chem. Soc., vol. 114, 834-843(1992)). This implies a formation mechanism based on strongelectrostatic interactions and/or the ionic pairing between the oligomersilicate precursor and the structure engineering agent, making theremoval of the later difficult.

The discovery of the carbon fullerene structure (C₆₀) during the 80s,which consists of a hollow sphere whose wall is made up of sixty carbonatoms (Nature, vol. 318, 162-163 (1985)), led to a new materials era ofgreat discoveries as, for example, the carbon nanotubes (Nature, vol.354, 56-58 (1991)). These structures and/or the nanotubular morphologiespresent interesting physical and chemical properties, making themsuitable for the construction of nanoelectronic innovating devices,among other applications. Due to this, the synthesis of nanomaterials ofcarbon and inorganic materials has boomed in the past few years. In 1992the first nanotubes and/or structures fullerene type of MoS₂ and WS₂(Nature, vol. 360, 444-446 (1992)), were obtained. Since then a greatvariety of nanomaterials includes: inorganic oxides such as: VO₂, ZrO₂,TiO₂, SiO₂, Al₂O₃, ZnO and TeO₂, sulphides, selenides, telurides,nitrates and transition metal carbides; among others (Dalton Trans.,1-24 (2003)).

On the other hand, a series of studies in confined fluids (M.Lozada-Cassou et al. J. Chem. Phys., vol. 80, 3344-3349 (1984); J. Chem.Phys., vol. 92, 1194-1210 (1990); J. Chem. Phys., vol. 98, 1436-1450(1993); Mol. Phys., vol. 86, 759-768 (1995); Phys., Rev. E., vol. 53,522-539 (1996); Phys. Rev. Letts., vol. 77, 4019-4022 (1996); Phys. Rev.E., vol. 56, 2958-2965 (1997); Phys. Rev. Letts., vol. 79, 3656-3659(1997)), showed that the confinement and curvature at nano-scaleproduces electric fields and molecular strengths of outstandingintensity. These studies show, for example, that in nano-confinement aseparation of charge in the ionic fluid can occur (Phys. Rev. Letts.,vol. 79 656-659 (1997)), which implies confinement pressures in theorder of 1,250 atm and intermolecular repulsion forces of 5×10⁻⁹ N. Thisresult highlights the importance of the confinement for the molecularseparation and it oriented the present invention towards the search oftubular structures at nanometric scale and to the development of newmaterials with enhanced catalytic properties, semiconductive properties,etc.

The nanotubes are materials that are applied, for example, in processesinvolving adsorption phenomena, as they increase the contact area byexposing the internal surface, the external surface, the surface in thevertex and the surface in the interlayer regions that compose the walls.This together with the increase of the intensity in the force fields,due to curvature and confinement of the nanotubes, enhance the catalyticactivity of catalysts or of active phase materials supported onnanotubes. According to the porous materials classification, thenanotubes present mesopores which are homogeneous with a size between 1to 50 nm and with a high pore volume. These characteristics make thenanotubes potentially useful as catalytic supports or as catalysts.

In the past it has been possible to synthesize nanotubes with wallscomposed of zirconium oxide, alumina, titania with anatase structure,and transition metal sulfides among others, by means of methodsinvolving the addition of a structure engineering agent, consisting of acationic, anionic and/or neutral tensoactive agent. However, thetensoactive elimination through calcinations, leads in most cases to thecollapse of the nanotubular structure.

Other procedures in the nanotubes synthesis consist in the applicationof porous membranes, organic or inorganic, to guide the nanotubeformation; however, they are generally applied for the case of materialswhose structure is compact or tridimensional (3D). The materials withbidimensional structures (2D), like plates and/or sheets, can formunidimensional materials (1D) of the nanotube type and/or nanofibers, bythe direct bending and/or rolling of its structure, due to temperatureeffect, to pressure or to the application of an electric potential, etc.

Titanium oxide is commonly presented as a tridimensional structurematerial (3D) and it is basically used as a semiconductor material inthe construction of electronic and optoelectronic devices, in themanufacturing of pigments and coatings, as catalyst and/or catalystsupport in several processes, as photocatalyst in the degradation oforganic compounds during environmental protection processes, asphotosensitive material in the construction of fuel cells and solarcells, etc.

Titanium dioxide is known to exist in three crystalline phases, anatase,rutile and brookite, as well as an amorphous phase. There are otherphases but these ones are the most common. The anatase and rutile phaseshave a tetragonal crystal lattice, and the brookite phase has anorthorhombic crystal lattice or structure. This information is wellknown in the area. The anatase and rutile phases which have a tetragonalcrystal lattice are different even though they both have a tetragonalcrystal lattice. The differences stem from the position of the atoms,the surroundings of the atoms, the lattice parameters, and the spacegroup inside the tetragonal crystal lattice, and because theseparameters are different these two phases are differentiated withdifferent names (anatase and rutile). Each phase presents differentproperties and among all of the phases anatase is the one that has mostapplications, due to the fact that it can be obtained easily through aconventional chloride or sulfide process.

On the other hand, nanotubes and/or titanium oxide nanofibers with theanatase structure have been obtained, improving in this way the texturalproperties of the titanium oxide. In this direction, published U.S.Application No. 2004/0265587 describes a procedure to obtain tubularTiO₂ particles with the anatase structure, with an external diameter of5 to 40 nm, with lengths of 50 to 1,000 nm and a specific area of 450m²/g if only one hydrothermal treatment is carried out and a specificarea in the range of 400 m²/g to 500 m²/g, if two hydrothermaltreatments are carried out; thus in general the synthesis requires twostages of hydrothermal treatment which involves an alkaline metal and anorganic alkaline base. The inventors apply such tubular particles asphotocatalysts and/or materials for the construction of photoelectriccells showing good results.

U.S. Pat. No. 6,537,517 refers to a process for titanium oxideproduction with tubular morphology and anatase structure, with orwithout the presence of silicon oxide, by means of a hydrothermictreatment involving an alkaline metal hydroxide. The TiO₂ nanotubes withanatase structure present specific surface areas between 200 and 500m²/g. It has been published in the literature (Ma, R.; Bando, Y.;Sasaki, T. Chemical Physics Letters, 2003, 380, 577-582) that it ispossible that the so claimed anatase nanotubes in the aforementionedpatent might have a lepidocrocite-type structure instead of the soclaimed anatase structure. The lepidocrocite structure is defined for aniron oxide compound (iron (III) oxide hydroxide, also known asγ-(FeOOH)). A lepidocrocite-type structure would mean that the soclaimed anatase-TiO₂ nanotubes would have the same structure as the iron(III) oxide hydroxide and the same space group; however, the cellparameters cannot be exactly the same neither the atoms positionsbecause in one case the titanium atom is involved and in the other casethe iron atom is involved. Thus it is clear that the state of the art isthat the crystalline structure, space group(s) and atomic positions inthe unit cell that composes the so claimed nanotubular anatase-TiO₂structure is not known.

A synthesized nanostructure with a phase different to anatasa is givenin the Korean laid-open patent application No. P2003-0026268 where thesynthesis of nanoparticles (balls or spherical crystal with a nanometricsize), mostly with the brookite phase, which is known to have anorthorhombic crystal lattice, and some rutile phase, with tetragonalcrystal lattice, is reported. The starting materials for the synthesisare TiCl₄ and HNO₃.

In the case of the U.S. Pat. No. 6,537,517 the starting material is apowder of crystalline titanium oxide (crystalline titania powder withthe anatase or rutile phase) with an average particle size between 2 to100 nm, preferably from 2 to 30 nm (the size of the crystallites thatcompose the particles is not provided). The starting material issubjected to a hydrothermal treatment, in the presence of an alkalimetal hydroxide, that comprises one step. However, as it is alreadymentioned in the published U.S. Application No. 2004/0265587, the use ofa titania powder as starting material does not produce a high yield ofthe titania nanotubes with anatase phase. On the contrary sphericalparticles are synthesized in a higher yield than the nanotubes and thefinal product presents a large residual amount of sodium that hindersthe efficiency of the nanotubes as possible catalyst. Also in U.S. Pat.No. 6,537,517 it is mentioned that the nanotube titania obtained fromthe alkali hydrothermal treatment may further be heat-treated at from200 to 1,200° C. to improve the crystallinity of TiO₂ and to increasethe catalytic activity and that the nanotube does not collapse throughthis heat treatment. It is not mentioned how the heat treatment isperformed. It is assumed that it was done as a regular well known heattreatment which would involve a static, non-dynamic air atmosphere byplacing the product in an oven. It is claimed that the heat treatment isexpected to improve the crystallinity and activity of the nanotubes.However, there is no table or data comparing the properties of thenanotubes before and after such heat treatment. Very recently it hasappeared in the literature a paper entitled “Regulation of the PhysicalCharacteristics of Titania Nanotube Aggregates Synthesized fromHydrothermal Treatment” (Chien-Cheng Tsai and Hsisheng Teng, Chemistryof Materials 2004, 16, 4352-4358) where the precursor used is acommercial TiO₂ powder with a composition 70% anatase and 30% rutile,and a primary particle size of 21 nm (same method of synthesis as thereported in U.S. Pat. No. 6,537,517). In this paper the authors studyhow the stability and pore structure (surface area) of the obtainednanotubes vary with subsequent calcination at different temperatures(they do not give any specifics about the calcination procedure, thus itis assumed to be static air in an oven). The authors of the mentionedliterature paper found that the as-synthesized anatase nanotubes remaintube-like at 400° C. but these nanotubes have a sharp surface areadecrease with the calcination temperature sintering (collapsing) at 600°C. to form anatase rodlike structures. Subsequently the rodlikestructure agglomerates at 800° C., forming anatase cylindricalparticles, and at 900° C. these particles go through a phasetransformation to the rutile phase. These results contradict use ofthermal treatment over the interval (200° C. to 1200° C.) in U.S. Pat.No. 6,537,517 to improve crystallinity without collapse or phasetransformation at the high temperature.

In published U.S. Application No. 2004/0265587, titanium oxide sol isused as a starting material in which particles (no powders) withspecific average particle diameters (2 to 100 nm, preferably 5 to 80 nm)are dispersed in water to prepare a water dispersion sol which is usedas starting material. The synthesis method outlined in published U.S.Application No. 2004/0265587 to obtain tubular titanium oxide particlesinvolves preparing the water dispersion sol (this step requires heatingand many steps). Then the water dispersion sol of titanium oxideparticles is subjected to a one step of hydrothermal treatment followedby washing and calcining, or the water dispersion sol of titanium oxideparticles is subjected to a two hydrothermal treatments instead of one.The first hydrothermal treatment is carried out in the presence of analkali metal hydroxide together with ammonium hydroxide and/or anorganic base. The presence of the ammonium hydroxide and/or an organicbase is claimed to reduce the alkali metal impurities in the tubulartitanium oxide particles. The hydrothermal treatment is carried out attemperatures between 80 to 250° C. (which is a higher temperature thanthe required in the U.S. Pat. No. 6,537,517). While in the secondhydrothermal treatment the presence of a cation is required and thetemperature is the same as in the first hydrothermal treatment. Thesynthesis presented in published U.S. Application No. 2004/0265587involves many steps, many reactants, high temperatures and possibly asecond hydrothermal treatment, and consequently the method becomesindustrially of high cost; also high temperatures are required in bothhydrothermal treatments. The method in published U.S. Application No.2004/0265587 also involves a heating treatment (named as “reductiontreatment”) in an inert gas atmosphere, under reduced pressure or in areducing gas atmosphere. It is not said if this so called reductiontreatment is done in a dynamic flow or static flow of the gas thatcomposes the atmosphere, thus it is assumed that it is carried out in astatic way. The formula of the final product given in published U.S.Application No. 2004/0265587 includes nitrogen and another transitionmetal different to titanium in the case of preparing mixed metalcompounds or M=Ti if not mixed metal synthesis is carried out. The givenformula of the claimed synthesized tubular titanium oxide particles withanatase, or rutile, or brookite phase is Ti_(a)M_(b)O_(x)N_(y).Experimental evidence on all the cell parameters to support theindication that the tubular titanium oxide particles have anatase, orrutile, or brookite phase is not provided in published U.S. ApplicationNo. 2004/0265587.

SUMMARY OF THE INVENTION

The present invention relates to nanomaterials of titanium oxide(TiO_(2-x), where 0≦x≦1), which have as a building block a crystallinestructure with an orthorhombic symmetry described by at least one of thespace groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb. The positions of theatoms, their surroundings, the crystal parameters and space groups inthe crystalline materials of the present invention do not match any ofthe known phases of titanium dioxide (anatase, brookite, rutile) or thenot so well known phases (beta and others). The new crystallinestructure forms the basic unit of construction of nanomaterials whichare nanofibers, nanowires, nanorods, nanoscrolls and/or nanotubes. Thenanomaterials are obtained from a precursor that is isostructural to thenew crystalline structure and is a hydrogen titanate and/or a mixedsodium and hydrogen titanate. These titanates are the hydrogenated,protonated, hydrated and/or the alkalinized phases of the newcrystalline structure. The new crystalline structure is named the “JTphase” of the TiO_(2-x), The term “JT phase” as used herein means acrystalline structure or crystalline phase with an orthorhombic symmetryhaving the formula TiO_(2-x), wherein 0≦x≦1, and has at least one of thespace groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.

Also, this invention further involves the procedure of synthesis of thenanomaterials, which have as a building block the new JT phase. The newnanomaterials after being subjected to dynamic thermal treatment in aninert, oxidizing or reducing atmosphere have a stable nanotubularstructure that is preserved intact without collapse, thus maintainingtheir fibrilar morphology and high specific area. It has been discoveredthat the hydrogen titanate and mixed sodium and hydrogen titanatenanotubes, that are unstable when heated higher than 300° C. in air inan oven can be stabilized by heating in a dynamic oxidizing, inert orreducing flowing atmosphere, which transforms the titanate nanotubesinto the stable, oxygen deficient JT phase nanotubes.

The nanomaterials of the present invention are useful mainly as asupport for catalysts and/or as catalysts, as photocatalysts, asadsorbents, as semiconductors in the construction of electronic devices,in photoelectric cells, in pigments and cosmetics, among otherapplications.

Thus, one aspect of the present invention relates to nanomaterials oftitanium oxide (TiO_(2-x), where 0≦x≦1), which have as building block acrystalline structure with an orthorhombic symmetry described by atleast one of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb. Thenew crystalline structure is the basic unit of construction ofnanomaterials which are nanofibers, nanowires, nanorods, nanoscrollsand/or nanotubes. The nanomaterials are obtained from a precursor thatis isostructural to the new crystalline structure. The precursor is ahydrogen titanate and/or a mixed sodium and hydrogen titanate. Thesetitanates are the hydrogenated, protonated, hydrated and/or thealkalinized phases of the new crystalline structure.

Another aspect of this invention relates to a procedure for thesynthesis of the nanomaterials, which involves among other stages: analkaline treatment of the starting materials followed by a thermaltreatment under either reflux conditions or hydrothermal treatment atatmospheric pressure, controlled or autogenous and continuous stirring;then a treatment of ionic exchange is carried out, which can be done inaqueous media with different levels of acidity or in alcoholic media orin aqueous media, to obtain the hydrogen titanates or the mixed sodiumand hydrogen titanates. Finally, the titanates are subjected to adynamic thermal treatment stage in an inert, oxidizing or reducingatmosphere to produce a nanomaterial with an orthorhombic latticedescribed by at least one of the space groups 59 Pmmn, 63 Amma, 71 Immmor 63 Bmmb.

A further aspect of the present invention is to provide a procedure tosynthesize the nanomaterials with the structure TiO_(2-x) JT phase,where 0≦x≦1, and that presents a fibrilar morphology and/or a tubularmorphology depending on the general conditions during the synthesisprocedure.

A further aspect of the present invention is to provide nanomaterials oftitanium oxide (TiO_(2-x), where 0≦x≦1), which have a new crystallinestructure and are obtained from an isostructural precursor composed ofhydrogen titanate and/or a mixed sodium and hydrogen titanate whichcorrespond to the hydrogenated, protonated, hydrated and/or thealkalinized phases of the new crystalline structure.

Another aspect of the present invention is to provide nanomaterials oftitanium oxide (TiO_(2-x), where 0≦x≦1), which have a new crystallinestructure that presents a fibrilar morphology or nanotubular morphologyobtained by a process which comprises an stage of thermal treatment in acontrolled dynamic atmosphere composed of air, O₂, N₂, He, Ar or amixture thereof at any concentration or in a controlled dynamicatmosphere composed of a mixture of inert gas and H₂ in a concentrationof 5% or 30% by volume of H₂ based on the total volume of gas, in adynamic heating regime starting from an isostructural precursor composedof hydrogen titanate and/or a mixed sodium and hydrogen titanate.

BRIEF DESCRIPTION OF THE DRAWINGS

With the objective of obtaining an understanding of the crystallinestructure, which is the building block of the nanomaterials of titaniumoxide of the present invention, called TiO_(2-x) JT phase; where xvaries between 0 and 1 due to the fact that this crystalline structurepresents a high oxygen deficiency, and is described by at least one ofthe space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb, reference is madeto the following Figures:

FIGS. 1 a, 1 b and 1 c are scanning electron microscopy images (SEM)where it is observed the nanofibrilar and/or nanotubular morphology ofthe hydrogen titanate, of the mixed sodium and hydrogen titanate and ofthe TiO_(2-x) JT phase, where 0≦x≦1 in which:

FIG. 1 a shows nanofibers and/or nanotubes obtained at low temperature,from 50 to 130° C.,

FIG. 1 b shows nanofibers and/or nanotubes obtained at mediumtemperature, from 130 to 160° C. and

FIG. 1 c shows nanofibers and/or nanotubes of TiO_(2-x) JT phase, where0≦x≦1, obtained after dynamic thermal treatment of the hydrogentitanates, or the mixed sodium and hydrogen titanates synthesized athigh temperature, between 160 and 180° C.;

FIGS. 2 a, 2 b and 2 c are X-ray dispersive energy spectra (EDX) whichshow the quantitative chemical composition of the nanotubes and/ornanofibers, a) hydrogen titanate, b) mixed sodium and hydrogen titanateand c) TiO_(2-x) JT phase, where 0≦x≦1, respectively;

FIGS. 3 a-3 f show transmission electron microscopy (TEM) images whereFIGS. 3 a, 3 b and 3 c correspond to nanotubes of hydrogen titanates andmixed sodium and hydrogen titanates, and FIGS. 3 d-3 f correspond tonanotubes of TiO_(2-x) JT phase, where 0≦x≦1. FIGS. 3 a-3 f areillustrative examples of the typical transmission electron microscopyimages obtained for the present hydrogen titanate and/or mixed sodiumand hydrogen titanate and the TiO_(2-x) JT phase, where 0≦x≦1.

FIG. 4 shows the X-ray diffraction patterns: (a) titanium compound,meaning TiO₂ with anatase structure or phase; (b) and (c) hydrogentitanates and mixed sodium and hydrogen titanates synthesized at 100° C.(b) and at 160° C. (c), respectively; which are precursors of theTiO_(2-x) JT phase, where 0≦x≦1. FIG. 4 is an illustrative example ofthe typical X-ray diffraction patterns obtained for the hydrogentitanate and/or mixed sodium and hydrogen titanate. The intensity of thepeaks might change but not their position;

FIG. 5 shows the characteristic nitrogen adsorption isotherm obtainedfor the hydrogen titanate nanomaterials, for the mixed sodium andhydrogen titanate and for the TiO_(2-x) JT phase, where 0≦x≦1;

FIG. 6 shows the characteristic pores distribution of the hydrogentitanate nanostructures, of the mixed sodium and hydrogen titanate andof the TiO_(2-x) JT phase, where 0≦x≦1;

FIG. 7 shows the X-ray diffraction pattern of the TiO_(2-x) JT phase,where 0≦x≦1, which is object of the present invention;

FIG. 8 shows the model of the formation of the tubular structuresstarting from an inwards rolled layer and/or sheet;

FIGS. 9 a and 9 b show the basic structural crystalline cell or unit ofthe TiO_(2-x) JT phase, where 0≦x≦1. The crystalline unit has anorthorhombic symmetry, and is described by at least one of the spacegroups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb, and it is the buildingblock of the nanomaterials, which are obtained from the synthesisprocedure of the present invention. FIG. 9( a) shows a three-dimensional(3D) view of the unit cell. FIG. 9( b) shows a bi-dimensional (2D) viewof the unit cell in the (a, b) crystalline plane. As indicated, thepositions of the atoms, their surroundings, the crystal parameters andspace groups in our materials do not match with any of the knowntitanium dioxide phases (anatase, brookite, rutile, amorphous) or withthe not so well known phases (beta and others);

FIG. 10 a illustrates schematically: (a) the cell parameters of the unitcell of the TiO_(2-x) JT phase, where 0≦x≦1 with tubular structure whichis obtained from the synthesis procedure of the present invention; FIGS.10 b and 10 c, respectively, present the X-ray diffraction patternssimulated for the model presented in FIG. 10( a) and for the structuresshown in Tables 2 and 9 of the titanates (hydrogen titanate and/or mixedsodium and hydrogen titanate), and of TiO_(2-x) JT phase, where 0≦x≦1,respectively;

FIGS. 11 a-11 d show the following images: FIG. 11 a—transmissionelectron microscopy (TEM) image, which presents the morphology of thenanotubes of TiO_(2-x) JT phase, where 0≦x≦1; FIG. 11 b—experimentalelectron diffraction pattern of the nanotube or nanofiber presented inFIG. 11 a and where the reflections (200) and (020) associated with cellparameters a and b respectively are shown as well as the angle formed;FIG. 11 c—the theoretically calculated electron diffraction pattern, forthe theoretical model of the JT phase, with orthorhombic structure thatis presented in FIGS. 9 a and 9 b; FIG. 11 d—experimental electrondiffraction pattern of a nanotube where the reflections (001) and (020)associated with cell parameters c and b respectively are shown, as wellas the angle formed;

FIG. 12 shows a high resolution transmission electron microscopy (HRTEM)of a fiber and/or tube where an inter-planar spacing of approximately0.7 nm is observed and it is associated with the c cell parameter of theorthorhombic unit cell of the TiO_(2-x) JT phase, where 0≦x≦1; and

FIG. 13 illustrates a dynamic heating system for forming thenanomaterial TiO_(2-x) JT phase, where 0≦x≦1, starting from the dynamicthermal treatment of the hydrogen titanates and/or the mixed sodium andhydrogen titanates of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The nanomaterials of titanium oxide of the present invention have as abuilding block a crystalline structure with an orthorhombic symmetry,and are referred to as the JT phase of the TiO₂, where 0≦x≦1 andpreferably x is between 0.1 and 0.95. The new crystalline structure isthe basic unit of construction of nanomaterials which are nanofibers,nanowires, nanorods, nanoscrolls and/or nanotubes. The nanomaterials areobtained from a precursor that is isostructural to the new crystallinestructure. The precursor is a hydrogen titanate and/or a mixed sodiumand hydrogen titanate. These titanates are the hydrogenated, protonated,hydrated and/or the alkalinized phases of the new crystalline JT phase.In the structure of the TiO_(2-x) JT phase, x can vary form 0 to 1 dueto the fact that the structure has an oxygen deficiency, which makes ita useful material for applications that involve surface adsorptionphenomenon, among other applications.

Likewise, this invention is related to the procedure of synthesis of thepresent nanomaterials, which involves among other stages: an alkalinetreatment of the starting materials followed by a thermal treatmentunder either reflux conditions or hydrothermal treatment at atmosphericpressure, controlled or autogenous; then a treatment of ionic exchangeis carried out, which can be done in alcoholic and/or in aqueous mediaor in aqueous media with different levels of acidity, to thus obtainingthe hydrogen titanates or the mixed titanates of hydrogen and sodium.Finally, the titanates are subjected to a dynamic thermal treatmentstage in an inert, oxidizing or reducing atmosphere to producing amaterial with an orthorhombic lattice denominated as the JT phase of theTiO_(2-x), where 0≦x≦1.

The calculated cell parameters of the unit cell of the TiO_(2-x) JT,where 0≦x≦1, are reported in Table 9 and they vary between the followingintervals: a from 0.283 to 0.324 nm, b from 0.354 to 0.395 nm and c from0.695 to 0.735 nm, for the case of the 59 Pmmn space group, and from1.408 nm to 1.453, for the case of the 63 Amma, 71 Immm, and 63 Bmmbspace groups; with α=β=γ=90°. The cell parameters depend on the generalsynthesis conditions of the hydrogen titanates and/or the mixed sodiumand hydrogen titanates and on the conditions during the dynamic thermaltreatment of them, under a controlled dynamic atmosphere and atemperature in the range of 200° C. and 500° C., preferably between 200°C. and 450° C., and most preferably between 200° C. and 400° C.

In the present invention, the unit cell of TiO_(2-x) JT, where 0≦x≦1,(FIGS. 9 a and 9 b) represents the basic unit of construction of theplates and/or sheets that compose the nanotubes, nanofibers, nanowires,nanorods and/or nanoscrolls (FIGS. 8 and 10 a).

The nanomaterials with the TiO_(2-x) JT structure, where 0≦x≦1, presenta nanofibrilar morphology and/or a nanotubular morphology, which isconstituted of piled structural layers that are rolled inwards (FIG. 8)into themselves. These layers present a great deficiency of oxygen andtheir basic structural unit, which is represented in FIGS. 9 a and 9 b,consist of two oxygen atoms per one of titanium in a basic cell thatrepeats thus forming the piled sheets that compose the nanotubes,nanofibers, nanowires, nanorods and/or nanoscrolls by rolling inwardsinto themselves. The unit cell of the structure TiO_(2-x) JT, where0≦x≦1, presents an orthorhombic symmetry, which is described by severalspace groups (59 Pmmn, 63 Amma, 71 Immm, or 63 Bmmb) defined in the“International Tables for Crystallography” (International Tables forCrystallography Volumen A, Space-Group Symmetry. Theo Hahn, editor,Kluwer Academic Publisher: Netherlands, 1989). The atomic coordinatesare presented in Table 9.

The cell parameters of the phase called in this invention as JT, whosecell parameters and relative atomic coordinates do not match with any ofthe known titania phases, were determined experimentally bycharacterization techniques such as: the transmission electronmicroscopy (TEM), see FIG. 11 a. The electron diffraction patterns ofisolated nanofibers, see FIG. 11 b, were used to obtain the a and b cellparameters. The c parameter of the unit cell was obtained with highresolution transmission electron microscopy (HRTEM). An HRTEM image ispresented in FIG. 12. The observed experimental cell parameters area=0.317 nm, b=0.360 nm, and c=0.700 nm. The experimental results agreewith the same results obtained by theoretical simulation (see Table 9).In FIG. 11 c it is shown the theoretically calculated electrondiffraction pattern, for the theoretical model of the JT phase, withorthorhombic structure that is presented in FIGS. 9 a and 9 b, and thetheoretical pattern agrees well with the experimental electrondiffraction pattern presented in FIG. 11 b. From the experimentalelectron diffraction pattern presented in FIG. 11 b it can be seen thatthe angle formed between the cell parameters a and b is equal to 90°. InFIG. 11 d, which corresponds to an experimental electron diffractionpattern of a nanotube and where the reflections (001) and (020) areshown, it can be seen that the angle formed between the cell parametersb and c is equal to 90°. Thus the structure corresponds to a crystallinelattice which is orthorhombic and that has been confirmed by theoreticalmodeling using the structure presented in FIGS. 9 a and 9 b, whose cellparameters and angles agree with the experimental data (see Table 9).The positions of the atoms inside the orthorhombic lattice do not matchwith the position of the atoms in any known phase of titania.

The nanostructures and/or aggregates of nanotubes and nanofibers withthe TiO_(2-x) JT structure, where 0≦x≦1, present the followingcharacteristics: a pore diameter of 2 to 30 nm, 0.01 μm to 100 μm inlength and an specific area of 5 to 500 m²/g, preferably 100 to 400m²/g; depending on specific surface area of the titanate precursor used(hydrogen titanate and/or mixed sodium and hydrogen titanate).

The hydrogen titanate or titanium oxide hydrated and/or the mixed sodiumand hydrogen titanate, with the general formula HTiO₂ andNa_(x)H_(1-x)TiO₂, where 0≦x≦1; which are also aims of the presentinvention, have a structure with an orthorhombic symmetry similar to thestructure TiO_(2-x) JT phase, where 0≦x≦1. The structure of thesetitanates is described by several space groups (59 Pmmn, 63 Amma, 71Immm or 63 Bmmb) of the “International Tables for Crystallography”(International Tables for Crystallography Volumen A, Space-GroupSymmetry. Theo Hahn, editor, Kluwer Academic Publisher: Netherlands,1989) and their relative atomic coordinates are presented in Table 2,which correspond to the hydrogenated, hydrated, protonated and/oralkalinized phases. The calculated lattice parameters presented in Table2 agree well with the observed experimental data. The a b c parameter ofthe titanates unit cell (hydrogen titanate and mixed sodium and hydrogentitanate), which are precursors of TiO_(2-x) JT phase, were obtainedwith high resolution transmission electron microscopy (HRTEM). Theobserved experimental cell parameters for the hydrogen titanate and/ormixed sodium and hydrogen titanate are a=0.301 nm, b=0.378 nm, andc=0.735 nm. In general the lattice parameters in the hydrogen titanateand/or mixed sodium and hydrogen titanate are larger than the cellparameters of TiO_(2-x) JT phase, where 0≦x≦1. This is mainly due to thepresence of the hydrogen and/or sodium atoms which are located in theinter-sheet spacing (see figures in Table 2).

The relative atomic coordinates vary depending on both the concentrationof the alkaline metal in the mixed sodium and hydrogen titanate, and onthe crystalline array (see Table 2); which in turn depend on the generalsynthesis conditions. The cell parameters of the unit cell of theorthorhombic structure, that conforms the nanotubes and/or nanofibers ofthe hydrogen titanates and/or the mixed sodium and hydrogen titanate,vary within the following interval: a from 0.263 nm to 0.331 nm; b from0.332 nm to 0.448 nm, and c from 0.635 nm to 0.902 nm, for the case ofthe 59 Pmmn space group, and from 1.368 nm to 1.905, for the case of the63 Amma, 71 Immm, and 63 Bmmb space groups; with α=β=γ=90°. In the caseof the 59 Pmmn space group the c parameter of the unit cell is akin tothe spacing between the layers that constitute the walls of the nanotubeand/or nanoscroll. For the case of the 63 Amma, 71 Immm, and 63 Bmmbspace groups the interlayer spacing is related to the c cell parameterdivided by two, because in these cases the unit cell is constituted bytwo layers (see Table 2). The interplanar space ranges from 0.635 to0.902 nm for the case of the 59 Pmmn space group, and from 0.684 nm to0.953, for the case of the 63 Amma, 71 Immm, or 63 Bmmb space groups;with α=β=γ=90°.

Likewise, this invention is related with the procedure of synthesis ofthe titanate materials, which involves an alkaline treatment of thestarting materials followed by a thermal treatment under either refluxconditions, at atmospheric pressure, or hydrothermal treatment, atautogenous pressure (in an interval of 1 to 150 atm), with continuousstirring between 10 and 1,000 rpm, preferably 100-500 rpm, to obtain thehydrogen titanate and/or the mixed sodium and hydrogen titanate.Finally, the titanates are submitted to a stage of thermal treatment ina controlled dynamic atmosphere composed of air, O₂, N₂, He, Ar or amixture thereof at any concentration or in a controlled dynamicatmosphere composed of a mixture of inert gas and H₂ in a concentrationof 5% or 30% by volume of H₂ based on the total volume of gas,preferably in a dynamic atmosphere of air or nitrogen at a temperaturein the range of 200° C. and 500° C., preferably between 200° C. and 450°C.; most preferably between 200° C. and 400° C. to thus obtain thetitanium oxide material with the structure (JT), which is an aim or anobject of the present invention.

The method to obtain nanomaterials such as nanofibers, nanowires,nanorods, nanoscrolls and/or nanotubes of TiO_(2-x) JT phase, where0≦x≦1, produces a nanomaterial with high specific area, in an intervalof 5 to 500 m²/g.

Initially, the process comprises treatment of a titanium compound suchas: titanium oxide with a crystalline structure of the anatase type,amorphous titanium oxyhydroxide and/or amorphous titanium hydroxide,titanium oxide with a crystalline structure rutile type and/or directlyfrom the mineral called rutile and/or ilmenite; with an alkalinesolution in a concentration inside the range of 1 to 50 M, preferably of5 to 20 M, of sodium hydroxide or sodium carbonate, potassium hydroxideor ammonium hydroxide; preferably of sodium hydroxide or potassiumhydroxide. The NaOH/TiO₂ molar ratio used is in the range of 7 to 70 M,preferably 10 to 60 M, which includes a ratio H₂O/TiO₂ of 50 to 410 M,preferably 80 to 300 M.

The resulting suspension can be subjected to a treatment underhydrothermal conditions in a closed system without stirring or withcontinuous stirring between 10 and 1,000 rpm, preferably 100-500 rpm ata temperature in the range of 50° C. to 180° C., preferably between 80°C. and 160° C. under autogenous pressure in a range of 1 to 150 atmduring a time range of 1 to 100 hours, preferably of 1 to 80 hours.Alternatively, the first formed suspension may be subjected to a thermaltreatment under reflux conditions at atmospheric pressure without orwith continuous stirring between 10 and 1,000 rpm, preferably 100-500rpm; at a temperature in the range of 50° C. to 150° C., preferably inthe range of 80° C. to 120° C.; during a period of time in the range of1 to 100 hours, preferably in the range of 1 to 80 hours.

Next, the resulting materials are subjected to a treatment of ionicexchange with a diluted acid solution with a concentration in the rangeof 0.1 and 1 M in aqueous media, and using acids such as the following:chlorhidric acid, sulfuric acid, nitric acid, fluoric acid, boric acidand/or phosphoric acid, or ammonium chloride, ammonium carbonate, or anyammonium salt capable of exchanging sodium; preferably a solution ofchlorhidric acid or nitric acid; until the pH of the suspension isreduced to a value between 1 and 7, preferably 2 and 4. Then, thesuspension is aged for an interval of time between 1 to 24 hours,preferably 3 to 18 hours; at room temperature, approximately 20° C. Oncethe aging process is finished, the suspension is separated by afiltration procedure; the obtained solid is washed with sufficientdeionized water and it is dried in a stove with a temperature in therange of 60° C. to 120° C., preferably between 80° C. and 110° C.,during a time period of 4 to 24 hours, preferably 12 to 18 hours.

The ionic exchange treatments can also be done directly by severalsuccessive washings with water or with alcohol (ethanol, n-propanol,i-propanol, n-butanol, etc) or with mixtures of alcohol-water in anyproportion. Depending on the extent of the washing and/or the decreaseon the pH of the suspension, the hydrogen titanates and/or the mixedsodium and hydrogen titanates, which are aims of the present invention,are produced.

The hydrogen titanates and/or the mixed sodium and hydrogen titanates,which are the precursors of the TiO_(2-x) JT phase, where 0≦x≦1, andaims of the present invention, present: disordered mesoporosity, averagepore diameter in the range of 3 to 25 nm, and a specific area in therange of 100 to 600 m²/g.

The specific area of the hydrogen titanates and/or the mixed sodium andhydrogen titanates, depends on their radial and longitudinal dimensions;the general synthesis conditions during the hydrothermal treatment orthe thermal treatment under reflux conditions; and on the prevailingconditions during the ionic exchange treatment.

With the same titanium compound, such as the TiO₂ with anatasestructure, and at constant hydrothermal reaction time and in a closedsystem, the synthesis carried out at reaction temperatures lower than150° C. (with a concentrated solution of an alkaline metal hydroxideand/or an alkaline metal carbonate in the range of 1 to 50 M, preferablyfrom 5 to 20M) produces small nanofibers and/or nanotubes with a pilinglevel of 1 to 10 layers. The longitude of the nanofibers and/ornanotubes vary between 0.1 and 1 μm. Whereas the synthesis carried outat a temperature between 150° C. and 180° C. produce nanofibers and/ornanotubes of higher dimensions, with piling levels between 10 to 50layers and a length between 1 and 50 μm.

Another parameter controlling nanofiber and/or nanotubes growth of thehydrogen titanates and/or the mixed sodium and hydrogen titanates, whichare the precursors of the TiO_(2-x) JT phase, where 0≦x≦1, and that areaims of the present invention, is the crystal size (size ofcrystallites) of the starting material, and the average particlediameter of particles composed of starting material crystalliteaggregates, used in the synthesis of the titanates. The average particlediameter used is in the range of 2.5 to 8 μm (2500 nm to 8000 nm). At asame reaction temperature and at same reaction timing for thehydrothermal reaction and or thermal reaction under reflux conditions,large crystalline aggregates of the starting material produce nanofibersand/or nanotubes of small dimensions, with piling levels of 1 to 10structural layers and a length between 0.1 and 5 μm.

A titanium starting material with small average particle diameter (<1μm) and/or with a small crystal size, such as between 3 and 10 nm,preferably 5 to 10 nm for anatase, produces large, stable (at roomtemperature) nanofibers and/or nanotubes of hydrogen titanate and/ormixed sodium and hydrogen titanates with lengths ranging from 1 to 10μm, and a piling level of 1 to 10 structural layers. According to thepresent invention, an anatase precursor crystal size of between 5 and 10nm is used in order to reach its complete transformation into nanotubesand/or nanofibers with dimensions in the nanoscale region, which exhibithigh specific surface area, and have a chemical composition of hydrogentitanate or mixed sodium and hydrogen titanate but not anatase orbrookite. If amorphous titania is used to form the titanate, a crystalsize of 0.1 nm to 5 nm, preferably 1 to 3 nm may be used.

Another parameter ruling the growth of the nanofibers and/or nanotubesof the hydrogen titanates and/or the mixed sodium and hydrogentitanates, which are the precursors of the TiO_(2-x) JT phase, where0≦x≦1, and that are aims of the present invention, is the time of thethermal reaction at reflux or hydrothermal conditions. Using the sameraw starting material, a reaction time lower to 6 hours produces smallnanofibers and/or nanotubes with a length between 50 to 500 nm, withstaking levels from 2 to 5 layers, while a reaction time between 7 and72 hours produces a nanofibers and/or nanotubes growth of 0.1 to 10 μmin length, and with staking levels from 1 to 10 layers.

The control of dimensions of the hydrogen titanate and/or the mixedsodium and hydrogen titanate is important, due to the fact that theycontrol the dimensions of the TiO_(2-x) JT phase, where 0≦x≦1, of thepresent invention.

The hydrogen titanates and/or the mixed sodium and hydrogen titanatessynthesized are subjected to a dynamic thermal treatment stage in anoxidizing, or reducing, or inert controlled atmosphere composed of air,O₂, N₂, He, Ar, or a mixture of them in any concentration, or in acontrolled dynamic atmosphere composed of a mixture of inert gas and H₂in a concentration of 5% or 30% by volume of H₂ based on the totalvolume of gas, preferably in an oxidizing or inert atmosphere such asair or nitrogen; in a dynamic flow of such gas, at a temperature between200° C. and 500° C., preferably between 200° C. or 300° C. and 450° C.;most preferably between 200° C. and 400° C., at a suitable flowing gasrate, such as between 0.1 to 1.0 liter per minute, preferably a flowinggas rate of 0.3 to 0.5 liter per minute to obtain and stabilize theTiO_(2-x) JT phase, where 0≦x≦1, and maintain the nanotubular morphologyand/or nanofibrilar morphology of the titanate precursor.

The surface area exposed by such titanate nanotubes generally drops whencalcined up to 300° C. when the calcination is carried out under asteady, non-dynamic, air atmosphere by placing the titanate nanotubes inan oven, and the nanotubes collapse to form nanoparticles of anatase.The dynamic heating method of the present invention provides a thermaltreatment methodology which avoids the sintering of the nanotubes, whilekeeping their nanotubular morphology and high surface area aftercalcination even at 400° C. The nanotubular structure is stabilizedafter the dynamic thermal treatment through the formation of the layeredstructure of the JT phase.

The dynamic thermal treatment may be conducted, for example, in asealed, tubular chamber 10, which is shown in FIG. 13, which maycomprise a tube of quartz or sintered silica in which a sample 11 of thetitanate nanotubes is placed and then sealed. Chamber 10 is connected byconduit 12, a mass flow controller 13, and conduit 15 to tank 14 whichcontains an oxidative gas, inert gas, or reducing gas supply. Conduit 16connects chamber 10 to water trap 18. Chamber 10 is surrounded byelectric heater 20. Dynamic flow during heating of the titanates isensured by observing bubbles in the water trap 18, which releases thegas via conduit 22.

The titanate nanotube sample 11 is heated to a temperature of 120° C. ata heating rate between 0.5° C. to 20° C./min, preferably between 1 to10° C./min. The sample remains at this temperature between 0.5 to 5hours, in order to slowly eliminate the absorbed water. Thereafter, thetemperature may be increased, for example, up to 400° C., at the sameheating rate. The sample remains at this temperature for between 1 to 24hours, preferably between 2 and 10 hours. The samples are heated under adynamic flow of an inert or oxidizing gas, such as a gaseous flow ofair, oxygen, nitrogen, helium, or argon, or any mixture of such gases inany concentration or in a controlled dynamic atmosphere composed of amixture of inert gas and H₂ in a concentration of, for example, 5 to 30volume percent of H₂ based on the total gas mixture. Preferably, air,nitrogen or helium is used.

The dynamic thermal treatment, under controlled atmosphere converts thehydrogen titanate and/or the mixed sodium and hydrogen titanate into theTiO_(2-x) JT phase, where 0≦x≦1; which maintains a unit cell withorthorhombic symmetry, that is described by the different space groupscontained in Table 9, according to the space groups description in the“International Tables for Crystallography” (International Tables forCrystallography Volumen A, Space-Group Symmetry. Theo Hahn, editor,Kluwer Academic Publisher: Netherlands, 1989). The cell parameters ofthe unit cell vary within the following intervals: a from 0.283 to 0.324nm, b from 0.354 to 0.395 nm and c from 0.695 to 0.735 nm, for the caseof the 59 Pmmn space group, and from 1.408 nm to 1.453, for the case ofthe 63 Amma, 71 Immm, and 63 Bmmb space groups; with α=β=γ=90° (seeTable 9); depending on the synthesis general conditions. Likewise, theresulting material has a pore diameter in the interval of 3 to 50 nm anda specific area of 5 to 500 m²/g, preferably from 100 to 400 m²/g; andit is characterized by tubular structures and/or fibrilar structureswith a length between 0.1 μm and 100 μm.

The textural properties, namely, the surface area, pore volume and porediameter, of the nanostructures of TiO_(2-x) JT phase, where 0≦x≦1,decrease as the temperature of the thermal treatment increases undercontrolled dynamic atmosphere; however, the observed nitrogen adsorptionand desorption isotherm is characteristic of the mesoporous nature ofthe nanotubes, which is confirmed through transmission electronmicroscopy as it is shown in FIGS. 3 d, 3 e and 3 f.

In FIGS. 1 a-1 c, the images from scanning electron microscopy (SEM) arepresented and where it is observed the nanofibrilar and/or nanotubularmorphology of the hydrogen titanate, of the mixed sodium and hydrogentitanate and of the TiO_(2-x) JT phase, where x The length of the fibersand/or tubes depends on the temperature of the hydrothermal or thermaltreatment reflux conditions; the higher the temperature the higher thedimension of the fibers and/or tubes. FIG. 1 a shows aggregates ofnanofibers and/or nanotubes of the hydrogen titanate and/or the mixedsodium and hydrogen titanate, which were obtained at low temperature,from 50° C. to 130° C. FIG. 1 b presents nanofibers and/or nanotubeswith dimensions between 1 and 50 μm, which were obtained at temperaturesbetween 130° C. and 160° C. FIG. 1 c displays the fibrilar and/ortubular morphology of the TiO_(2-x) JT phase, where 0≦x≦1, obtained bythe dynamic thermal treatment, between 300 and 500° C. and in controlleddynamic atmosphere, of the hydrogen titanates and/or mixed sodium andhydrogen titanates synthesized between 160 and 180° C. As it isobserved, the fibrilar morphology is kept.

In FIGS. 2 a-2 c, the X-ray dispersive energy spectra (EDX) arepresented. In FIG. 2 a, it is observed that the spectrum of thematerials that were not submitted to thermal treatment is basicallycomposed of Ti and O, with an O/Ti atomic ratio between 2.0 and 2.8,which suggests the presence of a hydrogen titanate. Likewise, in FIG. 2b, the spectrum of the mixed sodium and hydrogen titanate materialindicates that its composition is Ti, O and Na, with an O/Ti atomicratio between 2.0 and 2.8. In FIG. 2 c, the spectrum of the materialswhich were thermally treated in a dynamic oxidizing or reducing or inertatmosphere is shown. It is observed that the chemical composition is Tiand O, with an O/Ti atomic ratio between 1 and 1.9, which indicates ahigh oxygen deficiency.

In FIGS. 3 a-3 f, the transmission electron microscopy is shown, whereFIGS. 3 a, 3 b and 3 c correspond to nanotubes and/or nanofibers ofhydrogen titanates and/or mixed sodium and hydrogen titanates and FIGS.3 d, 3 e and 3 f correspond to nanotubes and/or nanofibers of TiO_(2-x)JT phase, where 0≦x≦1. In FIGS. 3 a and 3 d, it is observed that bothmaterials basically have nanotubular and/or nanofibrilar structures witha length of several micrometers, between 0.01 and 1 μm, and they havediameters between 2 nm and 15 nm and between 3 and 10 nm for thetitanates (hydrogen and/or mixed sodium and hydrogen titanate) and theTiO_(2-x) JT phase, where 0≦x≦1, respectively. The nanotubes aregenerally opened at the ends.

FIGS. 3 b and 3 e present the hollow transversal section of thenanotubes displaying an open internal spacing with diameters between 5and 15 nm. In FIG. 3 c, it is shown that the walls of the nanotubes ofhydrogen titanate and/or mixed sodium and hydrogen titanate are composedof approximately 1 to 5 structural layers. In FIG. 3 f, it is observedthat the nanotubes of the TiO_(2-x) JT phase, where 0≦x≦1, are as wellcomposed of 1 to 5 structural layers.

By comparing the images corresponding to the hydrogen titanate and themixed sodium and hydrogen titanate with the ones corresponding to theTiO_(2-x) JT phase, where 0≦x≦1 (FIGS. 3 a-3 f) it is established thateven after a dynamic thermal treatment in controlled atmosphere, at atemperature between 300° C. and 500° C.; the fibrilar morphology and/ornanotubular morphology of the materials is kept. Thus proving that thenanotubular and/or nanofibrilar structure obtained through theprocedure, aim of the present invention, is thermally stable, keepingits high specific area.

In FIG. 4, the X-ray diffraction patterns of the nanofibers, nanotubes,nanoscrolls, nanorods, nanowires and/or microfibers are displayed, whichcorrespond to the hydrogen titanate and/or mixed sodium and hydrogentitanate obtained under the preferred modalities of the presentinvention at two temperatures of synthesis. The intensity and theposition of the diffraction signal corresponding to the crystallinesurface (001) vary depending on the size of the fibers and/or tubes. Thespectrum label as (c) in FIG. 4 corresponds to an X-ray diffractionpattern of big fibers obtained at high temperature (160° C.), while thespectrum label as (b) in FIG. 4 corresponds to an X-ray diffractionpattern of small fibers obtained at low temperature (100° C.). Thesignals are broaden and less defined as the diameter and length offibers decrease. Also in FIG. 4, the X-ray diffraction pattern of atitanium compound used for the synthesis, meaning TiO₂ with anatasestructure, is shown in the spectrum label as (a), with the aim ofillustrating the structural changes that take place during thehydrothermal synthesis or thermal synthesis with reflux according withthe procedure which is aim of the present invention. It is clear fromthese X-ray diffraction patterns that the phase of the titanates is notanatase.

In FIG. 5, it can be seen that the nanostructures, nanofibers and/ornanotubes present a type IV isotherm (classification of theInternational Union of Pure and Applied Chemistry (IUPAC)), a histeresisis observed at a relative pressure (P/Po) of 0.4-0.6, indicating theexistence of mesoporous nanostructures. The isotherm presented in thisFigure is characteristic of both the hydrogen titanate and/or mixedsodium and hydrogen titanate and the TiO_(2-x) JT phase, where 0≦x≦1.Through the application of the BET method (Brunauer Emmett Teller) itwas determined that the nanotubes and/or nanofibers of both thetitanates (hydrogen titanate and/or mixed sodium and hydrogen titanate)and TiO_(2-x) JT phase, where 0≦x≦1, display a specific area between 100and 500 m²/g, which indicates that the obtained titanates (hydrogentitanate and/or mixed sodium and hydrogen titanate) of the presentinvention maintain their nanotubular structure and therefore theirmesoporosity and high specific area when are submitted to a dynamicthermal treatment in an atmosphere that can be oxidative, reductive orinert.

In FIG. 6, it is observed the porous distribution obtained from theapplication of the BJH mathematic model (Barrer Joyner Halenda) to thedesorption isotherms shown in FIG. 5, from which it is possible todetermine that the nanomaterials present a very homogeneous distributionof the pore size and in the range of 3 and 4 nm. The pore sizedistribution is associated to the internal diameter of the nanofibersand/or nanotubes, it was also determined in the transmission electronmicroscopy studies. Besides the nanomaterials, aims of the presentinvention, present a high pore volume, with a value between 0.3 and 1.5cm³/g.

FIG. 7 shows the X-ray diffraction pattern characteristic of theorthorhombic TiO_(2-x) JT structure, where 0≦x≦1, that comes from thehydrogen titanate and/or the mixed sodium and hydrogen titanatesubmitted to thermal treatment at a temperature between 300° C. and 500°C. and under a dynamic oxidizing, or reducing, or inert atmosphere.

FIG. 8 shows a schematic model of the nanotubes and/or nanoscrollsformation, which consists in the rolling and/or folding of laminarnanostructures having an overlapping of semitubular and semicircularstructures. It is believed that this mechanism illustrates the formationof the nanotubes of hydrogen titanates and/or mixed sodium and hydrogentitanates, which occurs during the procedure according to the presentinvention.

FIGS. 9 a and 9 b present the unit cell of the structure TiO_(2-x) JT,where 0≦x≦1, constituted of oxygen and titanium, with orthorhombicsymmetry that constitutes the basic unit of construction of the layersand/or sheets that roll and/or fold to form the nanotubes and/ornanoscrolls, by means of the mechanism illustrated in FIG. 8. FIG. 9 ashows a tridimensional view of the unitary cell and FIG. 9 b shows abidimensional view along the a,b crystalline plane.

In FIG. 10 a, a nanotube and/or nanoscroll is presented and built withthe unit cell of TiO_(2-x) JT phase, where 0≦x≦1, presented in FIGS. 9 aand 9 b; by following the formation model mechanism of nanotubes andnanoscrolls presented in FIG. 8. Also schematically illustrated is thespatial position of the unit cell and its cell parameters in thenanotubes or nanoscrolls. In the case of the 59 Pmmn space group the cparameter of the unit cell is akin to the spacing between the layersthat constitute the walls of the nanotube and/or nanoscroll. For thecase of the 63 Amma, 71 Immm, and 63 Bmmb space groups the interlayerspacing is related to the c cell parameter divided by two, because inthese cases the unit cell is constituted by two layers (see Table 9).FIGS. 10 b and 10 c present the X-ray diffraction patterns simulated forthe model presented in the FIG. 10 a and for the structures shown in theTables 2 and 9, respectively, which are characteristic of the nanotubesand/or nanofibers of the hydrogen titanates and/or of the mixed sodiumand hydrogen titanates as well as of the TiO_(2-x) JT phase, where0≦x≦1.

In FIGS. 11 a-11 d, the following images are shown: 11 a—transmissionelectron microscopy (TEM) image, which presents the morphology of thenanotubes of TiO_(2-x) JT phase, where 0≦x≦1; 11 b—experimental electrondiffraction pattern of the nanotube or nanofiber presented in FIG. 11 a;11 c shows the simulated electron diffraction pattern for thetheoretical model of the unit cell of the TiO_(2-x) JT phase, where0≦x≦1, that is presented in FIGS. 9 a and 9 b and that has to becompared with 11 b. The (200) reflection is associated with the a cellparameter of the unit cell and it has a value in the range of 0.283 nmand 0.324 nm. The (020) reflection is associated with the b cellparameter of the unit cell and it has a value in the range of 0.354 and0.395 nm. In addition, it is also observed that the nanofibers and/ornanotubes preferably grow in the b axis direction (see FIG. 10 a). 11 dshows the experimental electron diffraction pattern of a nanotube wherethe reflections (001) and (020) associated with cell parameters c and brespectively are shown, as well as the angle formed.

FIG. 12 shows a high resolution transmission electron microscopy (HRTEM)of a fiber and/or tube where an inter-planar spacing of approximately0.7 nm is observed, and that corresponds to the crystalline plane (001)of the structure of TiO_(2-x) JT phase, where 0≦x≦1. In the case of the59 Pmmn space group the c parameter of the unit cell is akin to thespacing between the layers that constitute the walls of the nanotubeand/or nanoscroll. For the case of the 63 Amma, 71 Immm, and 63 Bmmbspace groups the interlayer spacing is related to the c cell parameterdivided by two, because in these cases the unit cell is constituted bytwo layers (see Table 9). The interplanar space ranges from 0.695 to0.735 nm for the case of the 59 Pmmn space group, and from 0.704 nm to0.727, for the case of the 63 Amma, 71 Immm, or 63 Bmmb space groups;with α=β=γ=90°.

EXAMPLES

The following examples show production of titanium oxide nanomaterialswith the crystalline structure TiO_(2-x) JT phase, where 0≦x≦1, that arethermally stable and that are produced from a precursor of hydrogentitanate and/or mixed sodium and hydrogen titanate with orthorhombicstructure with our synthesis procedure. The molar ration NaOH/TiO₂ usedis in the range of 7 to 70 M, preferably 10 to 60 M, which includes aratio H₂O/TiO₂ of 50 to 410 M, preferably 80 to 300 M. The examples areillustrative of the present invention, and are not intended to limit thescope of the invention.

Examples 1 to 7

Examples 1 to 7 illustrate the preparation of the nanotubes of hydrogentitanate and/or mixed sodium and hydrogen titanate which areisostructural to the structure TiO_(2-x) JT phase, where 0≦x≦1, startingfrom titanium oxihydroxide (with a crystal size smaller than 3 nm, aspecific area of 190 m²/g, a pore volume of 0.22 cm³/g, and an averagepore diameter of 4.7 nm), prepared by the sol-gel method and under thesynthesis modality involving a hydrothermal treatment, aim of thepresent invention.

1.5 g of amorphous titanium oxihydroxide, obtained by the sol-gelmethod, is placed in contact with 100 cm³ of a 5 to 20 M alkalinesolution of sodium hydroxide. Each of the prepared suspensions inexamples 1 to 7 was poured in a closed vessel. In Table 1, the differenttemperatures used in the synthesis for each example are specified,within an interval from 100° C. to 180° C., and during a reaction timebetween 12 and 96 hours.

The resulting materials from each example is submitted to a ionicexchange treatment that involves a washing process with a 1M solution ofchlorhidric acid until the pH of the suspension lowers to a valuebetween 1 and 7, then all the solutions are aged for a period of timeranging from 12 to 18 hours. After the aging, each suspension isfiltered; then the thus obtained solids are washed with abundantdeionized water and dried at 110° C.

As it can be seen in FIGS. 1( a) and 1(b), the materials obtained arebasically constituted of nanofibers and/or nanotubes and/or aggregatesof nanofibers and/or nanotubes. In FIG. 4, lines (b) and (c), it can beobserved that the X-ray diffraction patterns correspond to a hydrogentitanate and/or to a mixed sodium and hydrogen titanate withorthorhombic lattice whose unit cell is described by the space groups 59Pmmn, 63 Amma, 71 Immm, or 63 Bmmb and the atomic positions presented inTable 2. The intensity and the position of the X-ray diffraction signalscorresponding to (001) surface, varies according to the fibers size andthe piling level among the nanotubes layers respectively. FIG. 4 is arepresentative X-ray diffraction pattern of the hydrogen titanate and/ormixed sodium and hydrogen titanate.

TABLE 1 Textural properties of the nanotubes of hydrogen titanate and/orthe mixed sodium and hydrogen titanates, obtained from amorphoustitanium oxihydroxide as starting material. Reaction Reaction PoreTemperature Time Area Pore Volume Diameter Example (° C.) (h) (m²/g)(cm³/g) (nm) 1 100 48 410 0.51 3.5 2 120 48 333 0.65 4.7 3 140 48 3420.71 5.0 4 160 48 278 0.74 6.7 5 180 48 44 0.42 21.4 6 100 12 272 0.374.0 7 100 96 43 0.15 6.7

As it can be seen in Table 1, depending on the temperature and thehydrothermal reaction time, the nanotubes and/or nanofibers present aspecific area between 40 and 500 m²/g and an average pore diameterbetween 2 and 25 nm. At high reaction temperatures, above 160° C.,Example 5, the specific area of the materials decreases due to thenanofibers and/or nanotubes growth; this same effect is produced with along reaction time, Example 7. The nanotubes are opened at the ends, asit can be seen in FIGS. 3 a-3 b, with an internal diameter between 3 and10 nm, and 1 to 50 layers with a spacing of 0.6 to 1.0 nm betweenlayers.

Examples 8 to 15

Examples 8 to 15 illustrate the preparation of the nanotubes and/ornanofibers of hydrogen titanate and/or mixed sodium and hydrogentitanate; starting from a titanium oxide (with anatase crystallinestructure, a crystal size of 8 nm, an average particle diameter between0.5 and 2.5 μm, a specific area of 102 m²/g, a pore volume of 0.51cm³/g, and a pore diameter of 11.3 nm) and using a synthesis procedurewhich involves a thermal treatment under reflux conditions, aim of thepresent invention.

7.5 g of titanium oxide with anatase structure is placed in contact with500 cm³ of a 5 to 20 M aqueous solution of sodium hydroxide. Each of theprepared suspensions in Examples 8 to 14 was submitted to a thermaltreatment, under reflux conditions, at a temperature between 80° C. and110° C. In Table 3, it is specified the reaction time and temperatureused in each example. The time interval is between 3 and 48 hours. Onlyin the case of the Example 15 the formed suspension, containing thetitanium precursor and the alkaline metal hydroxide solution, wassubmitted to a hydrothermal treatment at 160° C. in a closed systemunder autogenous pressure.

The resulting material from each example, are submitted to a ionicexchange treatment through a washing process with a 1 M solution ofchlorhydric acid until the pH of the suspension lowers to a valuebetween 1 and 7. Then all the solutions are aged for a period of timebetween 12 to 18 hours. Finally, each of the obtained suspensions isfiltered, and the thus obtained solids are washed with abundantdeionized water and dried at 110° C.

As it is observed in FIGS. 1 a and 1 b and in FIGS. 3 a, 3 b and 3 c, aswell as in the X-ray diffraction, FIG. 4 lines (b) and (c), the obtainedmaterials are basically constituted of nanofibers and/or nanotubes andtheir structure correspond to a hydrogen titanate and/or mixed sodiumand hydrogen titanate, which present an structural array withorthorhombic symmetry with a phase whose unit cell is described by thespace groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb and with the cellparameters and atomic positions presented in Table 2. FIG. 4 is arepresentative X-ray diffraction pattern of the hydrogen titanate and/ormixed sodium and hydrogen titanate and of the anatase precursor.

TABLE 3 Textural Properties of the nanotubes of hydrogen titanate and/ormixed sodium and hydrogen titanate obtained from anatase titanium oxideas starting material with a crystal size of 8 nm and a average particlediameter between 0.5 and 2.5 μm (500 nm to 2500 nm). Reaction ReactionPore Temperature Time Area Pore Volume Diameter Sample (° C.) (h) (m²/g)(cm³/g) (nm) 8 100 3 330 0.83 5.6 9 100 6 331 0.87 5.6 10 100 12 4010.93 5.3 11 100 24 314 1.0 5.9 12 100 48 198 0.61 6.5 13  85 48 301 0.736.2 14 110 6 287 0.64 5.6 15  160* 48 181 0.75 11.3 *Hydrothermaltreatment.

As it can be seen in Table 3, depending on the temperature and reactiontime fixed during the thermal treatment, the resulting materialspresent: a specific area between 150 and 500 m²/g, an average porediameter between 4 and 12 nm and a pore volume between 0.5 and 1.2cm³/g. The nanotubes are opened at the ends and have 1 to 50 layers witha spacing of 0.6 to 1.0 nm, between layers.

From Examples 8 to 12 in Table 3 it is observed that the optimal thermaltreatment time, under reflux conditions, is of 12 hours. A lower orupper time decreases the displayed specific area of the materials. Onthe other hand, it can be observed that for the experiments carried outat the same reaction time and at a temperature above 100° C., thespecific area decreases (Examples 14 compared with example 9); while thedecrease of the temperature at long reaction times, Examples 12 and 13,favors the specific area of the nanostructures.

Example 16

This example illustrates the preparation of nanotubes and/or nanofibersof hydrogen titanate and/or mixed sodium and hydrogen titanate startingfrom a titanium oxide compound with a rutile type crystalline structurewith a crystal size of 15 nm with an average particle diameter of 0.25μm.

1.5 g of a titanium oxide compound with a rutile type crystallinestructure is placed in contact with 100 cm³ of a 5 to 20 M aqueoussolution of sodium hydroxide. The formed suspension is submitted to ahydrothermal treatment at a temperature of 180° C. during 48 hours andunder autogeneous pressure in a closed system.

The resulting material is submitted to an ionic exchange treatment witha 1M solution of chlorhydric acid until the pH of the suspension lowersto a value between 1 and 7. Then the solution is aged for a period oftime between 12 and 18 hours. After aging, the suspension is filteredand the thus obtained solid is washed with abundant deionized water anddried at 110° C.

The material obtained in this example showed similar characteristics tothe materials obtained in the former Examples 1 to 15. It is basicallyconstituted of nanofibers and/or nanotubes FIGS. 1( a and b). Its X-raydiffraction pattern, FIG. 4 lines (b) and (c), shows that its structurecorresponds to a hydrogen titanate and/or to a mixed sodium and hydrogentitanate with orthorhombic structure whose unit cell is described by anyof the following space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb,reported in Table 2. FIG. 4 is a representative X-ray diffractionpattern of the hydrogen titanate and/or mixed sodium and hydrogentitanate.

The nanotubes and/or nanofibers are constituted by 1 to 50 layers,presenting a specific area between 50 and 100 m²/g and an average porediameter between 2 and 15 nm.

Example 17

This example illustrates the preparation of nanotubes of hydrogentitanate and/or mixed sodium and hydrogen titanate starting directlyfrom a rutile mineral with an average particle diameter between 20 and50 μm.

1.5 g of highly crystalline rutile mineral is placed in contact with 100cm³ of a 5 to 20 M aqueous solution of sodium hydroxide. The formedsuspension is submitted to a hydrothermal treatment, at a temperature of180° C., during 72 hours and under autogeneous pressure in a closedsystem.

The resulting material is submitted to an ionic exchange treatment witha 1M solution of chlorhydric acid until the pH of the suspension lowersto a value between 2 and 6. Then the solution is aged for a period oftime between 12 and 18 hours. After aging, the suspension is filteredand the thus obtained solid is washed with abundant deionized water anddried at 110° C.

The material obtained in this example showed similar characteristics tothe materials obtained in the former Examples 1 to 16. It is basicallyconstituted of nanofibers and/or nanotubes of FIGS. 1 a and 1 b. ItsX-ray diffraction pattern, FIG. 4 line-type (c), shows that itsstructure corresponds to a hydrogen titanate and/or to a mixed sodiumand hydrogen titanate with orthorhombic structure whose unit cell isdescribed by any of the following space groups: 59 Pmmn, 63 Amma, 71Immm or 63 Bmmb, reported in Table 2. We recall that FIG. 4 is arepresentative X-ray diffraction pattern of the hydrogen titanate and/ormixed sodium and hydrogen titanate and of anatase.

The nanotubes and/or nanofibers are constituted by several layers,presenting a specific area between 3 and 50 m²/g and an average porediameter between 2 and 15 nm.

Examples 18 to 20

Examples 18 to 20 illustrate the preparation of the nanotubes ofhydrogen titanate and/or mixed sodium and hydrogen titanate, startingfrom TiO₂ compounds with anatase structure with different crystal sizeand different average particle diameter and different texturalcharacteristics, which are presented in Table 4.

150 g of a TiO₂ compound with anatase structure, sample A to C in Table4, are placed in contact with 3 liters of a 5 to 20 M aqueous solutionof sodium hydroxide. Each of the formed suspensions, for each titaniumoxide sample with anatase structure from the Examples 18 to 20, issubmitted to a hydrothermal treatment at a temperature of 100° C. underautogeneous pressure in a closed system and during 24 hours.

The resulting material was put into contact with a 1 M solution ofchlorhydric acid, to perform the ionic exchange where the sodium isreplace by hydrogen, until the pH of the suspension lowered its valuebetween 1 and 7. Then the suspension is kept under constant stirring fora period of time between 1 and 24 hours. Finally, the suspension wasfiltered, it was washed with abundant deionized water and the solid wasdried at 110° C.

The resulting materials in each of the examples are basically composedof nanofibers and/or nanotubes, according to what is observed in FIGS. 1a and 1 b and FIGS. 3 a, 3 b and 3 c. The X-ray diffraction pattern,FIG. 4 lines (b), shows that the structure corresponds to a hydrogentitanate and/or to a mixed sodium and hydrogen titanate withorthorhombic structure and a unit cell described by the space groupspresented in Table 2. Also, in FIG. 4, it is observed that the intensityand the position of the X-ray diffraction peak that corresponds to the(001) plane, varies depending on the size of the fibers and depending onthe piling level between the layers of the nanotubes, which are ruled bythe textural and morphological characteristics of the TiO₂ used asstarting material. FIG. 4 is a representative X-ray diffraction patternof the hydrogen titanate and/or mixed sodium and hydrogen titanate andof anatase.

Under the same synthesis conditions the nanofibers and/or nanotubesgrowth depends on the textural and morphological characteristics of theTiO₂ compound used as starting material. The nanotubes are constitutedof 1 to 50 layers, with a space between layers of 0.6 to 1.7 nm. Thenanotubes are opened by the ends as it is observed in FIG. 3 a with aninternal diameter between 3 and 10 nm, presenting a specific areabetween 300 and 450 m²/g and an average pore diameter between 4 and 10nm, as it can be seen in Table 5.

TABLE 4 Textural properties of the titanium oxide with anatase structureused as starting material for the synthesis of the hydrogen titanatesand/or mixed sodium and hydrogen titanates. Average Particle or AverageAnatase Crystal aggregate Specific Average Pore Phase TiO₂ Size DiameterArea Pore Volume Diameter Compound (nm) (μm) (m²/g) (cm³/g) (nm) A 8 0.5a 2.5 102 0.42 16.5 B 8 2.5 a 8 101 0.37 14.5 C 5 <1 324 0.33 4.0

TABLE 5 Textural properties of titanium oxide with tubular structureobtained for Examples 18 to 20. TiO₂ Anatase Average Phase Pore CompoundSpecific Area Pore Volume Diameter Example Table 4 (m²/g) (cm³/g) (nm)18 A 316 0.72 9.1 19 B 389 0.70 7.2 20 C 401 0.77 7.7Conditions: 100° C., autogenous pressure, reaction time of 24 hours,exchange of 18 hours.

Example 21 to 25

Examples 21 to 25 illustrate the influence of the hydrothermal reactiontime in the synthesis of nanotubes and/or nanofibers of hydrogentitanate and/or mixed sodium and hydrogen titanate starting from a TiO₂compound with anatase phase whose characteristics are described in Table4, samples A to C.

In each case, 150 g of compounds A and C in Table 4, which are titaniumoxide with anatase structure, were put into contact with 3 liters of a 5to 20 M aqueous solution of sodium hydroxide. The suspension formed wassubmitted to a hydrothermal treatment, at a 100° C. temperature underautogenous pressure in a closed system during a period of time between 3and 14 hours.

The resulting materials of each example were put in contact with a 1Mchlorhydric acid solution, to exchange the sodium ions for hydrogen,until the pH of the suspension lowered to a value between 1 and 7. Thenthe suspension was left on constant stirring for a period of timebetween 1 and 24 hours. After aging, the suspension was filtered and theobtained solid was washed with sufficient deionized water and dried at110° C.

The resulting materials of each of the examples presented similarcharacteristics to the former Examples 1 to 25. They are basicallyconstituted of nanofibers and/or nanotubes, according to what isobserved in FIGS. 1 a and 1 b and FIGS. 3 a, 3 b and 3 c. In FIG. 4curves b, it is observed that the structure corresponds to a hydrogentitanate and/or to a mixed sodium and hydrogen titanate withorthorhombic structure, whose unit cell is described by the space groupsand atomic positions presented in Table 2. Also, in FIG. 4, it isobserved that the intensity and the position of the X-ray diffractionpeak that corresponds to the (001) plane, varies depending on the sizeof the fibers and depending on the piling level between the layers ofthe nanotubes. Thus, the growth of the nanotubes both in the radialdirection and the longitudinal direction is also ruled by the time ofthe hydrothermal reaction and the ionic exchange time with an acidsolution. FIG. 4 is a representative X-ray diffraction pattern of thehydrogen titanate and/or mixed sodium and hydrogen titanate and ofanatase.

The nanotubes are composed of 1 to 50 layers, with a 0.6 to 1.7 nm ofspacing between layers. The nanotubes are opened, as it can be seen inFIG. 3( b) with an internal diameter between 3 and 10 nm, presenting aspecific area between 380 and 470 m²/g and an average pore diameterbetween 5 and 8 nm, as can be observed in Table 6.

TABLE 6 Textural properties of the nanotubes of hydrogen titanate and/ormixed sodium and hydrogen titanates, obtained at different reactiontimes and ionic exchange according to examples 21 to 25. TiO₂ AnataseReaction Ionic Exchange Average Pore Average Pore Phase Compound TimeTime Specific Area Volume Diameter Example Table 4 (h) (h) (m²/g)(cm³/g) (nm) 21 A 3 1 414 0.78 7.6 22 A 3 18 450 0.72 6.4 23 A 6 18 3860.63 6.6 24 C 3 1 464 0.67 5.8 25 C 6 1 464 0.77 6.6Conditions: 100° C., autogenous pressure.

As it can be seen from Table 6, using the same starting material and atthe same temperature of synthesis, the growth of the nanofibers and/ornanotubes depends on the time of both the hydrothermal reaction and theionic exchange; the larger the hydrothermal reaction time, the biggerthe nanotubes size; while as the ionic exchange time is increased, withthe acid solution at a constant pH, the size of the nanofibersdecreases.

Examples 26 to 29

Examples 26 to 29 illustrate the synthesis of the nanotubes of hydrogentitanate and/or mixed sodium and hydrogen titanate starting from the Ato C compounds of Table 4, which comprise different compounds of TiO₂anatase phase, used as starting material, and a synthesis procedureconsisting of a thermal treatment under reflux conditions.

In each case, 150 g of titanium oxide with anatase structure, compoundsA to C in Table 4, were put into contact with 3 litters of a 5 to 20 Maqueous solution of sodium hydroxide. The suspension formed wassubmitted to a synthesis procedure consisting of a thermal treatmentunder reflux condition, at a temperature of 100° C. and at atmosphericpressure, in a system with continuous stirring between 10 and 1,000 rpm,preferably 100-500 rpm, during a 3 hour reaction period.

The resulting material was put in contact with a 1M solution ofchlorhydric acid to perform a sodium ionic exchange for hydrogen untilthe pH of the suspension lowers to a value between 1 and 7. Then thesuspension was aged for a 3 hour period for Examples 26 to 28 and for an18 hour period for the case of Example 29. After aging the suspensionwas filtered and the obtained solid was washed with sufficient deionizedwater and dried at 110° C.

The resulting materials of each of the examples presented similarcharacteristics to the former Examples 1 to 25. They are basicallyconstituted of nanofibers and/or nanotubes, according to what isobserved in FIGS. 1 a and 1 b and FIGS. 3 a, 3 b and 3 c. The X-raydiffraction patterns obtained are similar to those shown in FIG. 4,curve b, which correspond to the structure of the hydrogen titanate withorthorhombic structure, whose unit cell is described by the space groupsand atomic positions presented in Table 2. Also, in FIG. 4, it isobserved that the intensity and the position of the X-ray diffractionpeak that corresponds to the (001) plane varies depending on the size ofthe fibers and depending on the piling level between the layers of thenanotubes. The textural and morphological characteristics of thenanotubes resemble those of the TiO₂ used as starting material.

TABLE 7 Textural properties of the nanotubes of hydrogen titanate and/ormixed sodium and hydrogen titanate, obtained from different anatasecompounds using a procedure that involves thermal treatment under refluxconditions, with different ionic exchange time according to examples 26to 29. TiO₂ Anatase Reac- Ionic Average Average Phase tion ExchangeSpecific Pore Pore Exam- Compound Time Time Area Volume Diameter pleTable 4 (h) (h) (m²/g) (cm³/g) (nm) 26 A 3 3 414 0.83 8.0 27 B 3 3 3690.58 6.3 28 C 3 3 417 0.65 6.3 29 B 3 18 346 0.61 7.0Conditions: 100° C., autogenous pressure.

As it can be seen from Table 7, at constant synthesis conditions, thenanofibers and/or nanotubes growth depends on the textural andmorphological characteristics of the TiO₂ anatase phase used as startingmaterial, when the later is submitted to a thermal treatment underreflux conditions. The nanotubes are composed of 1 to 50 layers, with aspace between layers of 0.6 to 1.7 nm. The nanotubes are opened, as itcan be seen in FIG. 3 b with an internal diameter between 5 and 9 nm,presenting a specific area between 340 and 420 m²/g and an average porediameter between 2 and 10 nm. FIG. 3 is an example of a typical TEMimage of our hydrogen titanate and/or mixed sodium and hydrogen titanatematerial.

Examples 30 and 31

Examples 30 and 31 illustrate the synthesis of the nanotubes of hydrogentitanate and/or mixed sodium and hydrogen titanate starting from the Bcompound in Table 4. There is no need of acid treatment to perform theionic exchange of sodium by hydrogen.

150 g of the B compound in Table 4, which is a titanium oxide withanatase structure, was put in contact with 3 liters of a 5 to 20 Maqueous solution of sodium hydroxide. The suspension formed wassubmitted to a synthesis procedure comprising a hydrothermal treatmentunder reflux conditions, at a 100° C. temperature, autogenous pressurein a closed system, with continuous stirring between 10 and 1,000 rpm,preferably 100-500 rpm, during a period of 3 hours of reaction time.

In comparison with the former examples, in the case of Example 30 theresulting material was not put in contact with a chlorhydric acidsolution to perform the ionic exchange of sodium by hydrogen, but inthis case the solution was exhaustively washed with ethyl alcohol untilde suspension pH decreased to a value between 6 and 7. It was filtered,and dried at 110° C.

For the case reported in Example 31, the resulting material from thehydrothermal treatment stage was exhaustively washed with abundantbi-distilled water until the suspension pH decreased to a value between7 and 8, then it was filtered and dried at 110° C.

The resulting materials in each of the examples presented similarcharacteristics to the former Examples 1 to 29. They are basicallyconstituted of nanofibers and/or nanotubes according to FIGS. 1 a and 1b and FIG. 3 b. The obtained X-ray diffraction patterns are similar tothose presented in FIG. 4 line b, which shows that the structurecorresponds to a hydrogen titanate and/or to a mixed sodium and hydrogentitanate with orthorhombic structure whose unit cell is described by anyof the following space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb,reported in Table 2.

The nanotubes and/or nanofibers are constituted by 1 to 50 layers with aspace between layers of 0.6 to 1.7 nm. The nanotubes are opened at theend (FIG. 3 b) and they present an internal diameter between 3 and 10nm, a specific area between 180 and 310 m²/g and an average porediameter between 4 and 8 nm, as it can be seen in Table 8.

TABLE 8 Textural properties of the hydrogen titanate and/or mixed sodiumand hydrogen titanate obtained by the procedure of washings with wateror ethyl alcohol. Washing Specific Area Pore Volume Pore DiameterExample Solvent (m²/g) (cm³/g) (nm) 30 ethyl alcohol 303.96 0.544 7.1631 water 185 0.203 4.38

From Examples 30 and 31, it can be said that the ionic exchange ofsodium by hydrogen can be done with different washing solvents.

Examples 32 to 43

The nanotubes with hydrogen titanate structure obtained through theprocedures described in the former examples, were submitted to a thermaltreatment process at a temperature between 200° C. and 400° C. in andynamic oxidizing or inert or reducing atmosphere, thus obtaining thenanomaterial TiO_(2-x) JT phase, where 0≦x≦1, with an orthorhombicstructure whose structural characteristics are described by any of thefollowing space groups: 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb; and thecell parameters and atomic positions presented in Table 9.

The materials obtained in the Examples 32 to 43 present an orthorhombicstructure of TiO_(2-x) JT phase, where 0≦x≦1, aim of the presentinvention, and that keep the nanofibers and/or nanotubes morphology thatis present in the hydrogen titanates and/or mixed sodium and hydrogentitanates, which originated these materials after a thermal treatment ina dynamic oxidizing or inert or reducing atmosphere as it is shown inthe scanning electron microscopy images (SEM) in FIG. 1 c and FIGS. 3 d,3 e and 3 f.

Also, the nanotubes obtained after the thermal treatment in a dynamicoxidizing or inert or reducing atmosphere, present an orthorhombicstructure, with a crystalline phase named as the JT phase of titaniumoxide TiO_(2-x), where 0≦x≦1. The term “JT phase” means “a crystallinestructure or crystalline phase with an orthorhombic symmetry having theformula TiO_(2-x), wherein 0≦x≦1, and has at least one of the spacegroups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.” As it can be seen in FIG.7, the X-ray diffraction peak corresponding to the (001) plane is kept.This peak is also observed in the case of the precursor materials, FIG.4 curves b and c, which are the hydrogen titanate and/or the mixedsodium and hydrogen titanate.

Through the X-ray dispersive energy (EDX) spectrum presented in FIG. 2c, it is concluded that the nanostructures TiO_(2-x) JT phase, where0≦x≦1, present a high oxygen deficiency. In this spectrum is observedthat the chemical composition is Ti and O, with an O/Ti atomic ratiobetween 1 and 1.9, which indicates a high oxygen deficiency.

The materials with the orthorhombic structure, with a crystalline phaseTiO_(2-x) JT phase, where 0≦x≦1, aims of the present invention, presenta morphology of nanofibers and/or nanotubes as it can be observed inFIG. 1 c and FIGS. 3 d, 3 e and 3 f. The cell parameters of the phasecalled in this invention as JT, whose cell parameters and relativeatomic coordinates do not match with any of the known titania phases,were determined experimentally. The electron diffraction patterns ofisolated nanofibers, see FIGS. 11 a, 11 b and 11 d, were used to obtainthe a and b cell parameters. The c parameter of the unit cell wasobtained with high resolution transmission electron microscopy (HRTEM).An HRTEM image is presented in FIG. 12. In FIG. 12, the nanotubesobtained in the Examples 32 to 41 with orthorhombic structure are shown.An interlayer space of 0.7 nm that corresponds to the (001) plane andthat is related to the c parameter of the unit cell, which representsthe space between the layers that constitute the nanotube walls, asobserved in FIG. 10. The observed experimental cell parameters area=0.317 nm, b=0.360 nm, and c=0.700 nm. The experimental results agreewith the same results obtained by theoretical simulation (see Table 9).In FIG. 11 c it is shown the theoretically calculated electrondiffraction pattern, for the theoretical model of the JT phase, withorthorhombic structure that is presented in FIGS. 9 a and 9 b, and as itcan be seen the theoretical pattern agrees well with the experimentalelectron diffraction pattern presented in FIG. 11 b. From theexperimental electron diffraction pattern presented in FIG. 11 c it canbe seen that the angle formed between the cell parameters a and b isequal to 90°. In FIG. 11 d, which corresponds to an experimentalelectron diffraction pattern of a nanotube and where the reflections(001) and (020) are shown, it can be seen that the angle formed betweenthe cell parameters b and c is equal to 90°. Thus, the structurecorresponds to a crystalline lattice which is orthorhombic and that hasbeen confirmed by theoretical modeling using the structure presented inFIGS. 9 a and 9 b, whose cell parameters and angles agree with theexperimental data (see Table 9). The positions of the atoms inside theorthorhombic lattice of the JT phase do not match with the position ofthe atoms in brookite, which also has an orthorhombic lattice but has adifferent, single space group 61 Pbca, or in any other known phase oftitania.

The calculated cell parameters of the unit cell of the TiO_(2-x) JT,where 0≦x≦1, are reported in Table 9 and they vary between the followingintervals: a from 0.283 to 0.324 nm, b from 0.354 to 0.395 nm and c from0.695 to 0.735 nm, for the case of the 59 Pmmn space group, and from1.408 nm to 1.453, for the case of the 63 Amma, 71 Immm, and 63 Bmmbspace groups; with α=β=γ=90°. It is observed that the nanofibers and/ornanotubes grow preferably in the b axis direction. In Table 9 the atomspositions for each space group of the JT phase and its variations aregiven. All the information given in Table 9 compare very well with theexperimental data and as it can be seen the information given in Table 9do not match with any of the known phases of titania.

Also, the titanium oxide nanotubes with structure TiO_(2-x) JT phase,where 0≦x≦1, present a specific area between 100 and 400 m²/g, with adistribution of pore size that presents a pore average diameter between4 and 10 nm, as shown in Table 10. The titanium oxide nanotubes withTiO_(2-x) phase JT, where 0≦x≦1, are originated from the thermaltreatment, in a dynamic oxidizing or inert or reducing atmosphere, ofthe hydrogen titanates and/or mixed sodium and hydrogen titanates,keeping the nanotubular structure and the high specific area after thethermal treatment, which means that this material, the titanium oxidenanotubes with structure TiO_(2-x) JT phase, where 0≦x≦1, is thermallystable with potential applications as catalysts and/or as a catalystssupport and in other processes involving the adsorption phenomena.

From Examples 32 to 41 (see Table 10) it can be said that while theinitial area of the titanate precursor determines the area of the JTmaterial, an increase in the temperature lowers the JT area. In Examples32 and 33 the areas of the titanate starting materials are 342 m²/g and278 m²/g, respectively; whereas in Examples 34 through 39 the area ofthe titanate starting material is around 400m²/g, hence for thisexamples it is obtained a JT material with a larger area. In example 40the area of the titanate starting material is 369 m²/g, which is largerbut not much from that in Example 32. Nevertheless the difference in theJT area is quite important. This can be explained because the thermaltreatment was different. In example 32 the thermal treatment was done ina static oxidizing atmosphere. Examples 42 and 43 are originated fromthe thermal treatment in a dynamic reducing atmosphere composed of amixture of 5% or 30% by volume of H₂ in N₂, respectively; of thehydrogen titanates and/or mixed sodium and hydrogen titanates. As it canbe seen in Table 10 the starting materials for examples 42 and 43 werethe same used in examples 39 and 36 respectively. The only difference isthe dynamic atmosphere used and as it can be seen the use of a reducingatmosphere for thermal treatment can be successfully applied. Theobtained specific surface areas for examples 42 and 39 are quite similarand the same happens when comparing examples 43 and 36. In general dedifference in the area is approximately 10 m²/g less for the examplesunder dynamic reducing atmosphere.

TABLE 10 Textural properties of the nanotubes with structure TiO_(2−x)JT phase, where 0 ≦ x ≦ 1, prepared from the indicated examples andafter heat treatment. Example Precursor Tempera- Pore Pore Exam-(specific ture Time Area Volume Diameter ple area, m²/g) (° C.) (h)(m²/g) (cm³/g) (nm) 32^(b)  3 (342) 400 4 214 0.74 7.7 33^(b)  4 (278)400 4 127 0.44 9.2 34^(a) 19 (389) 200 4 348 0.73 6.3 35^(a) 19 (389)300 4 339 0.64 7.6 36^(a) 19 (389) 400 4 289 0.70 9.7 37^(a) 20 (401)400 4 319 0.54 4.8 38^(a) 20 (401) 400 4 323 0.54 4.8 39^(a) 26 (414)400 4 326 0.54 4.7 40^(a) 27 (369) 400 4 325 0.54 4.7 41^(c) 27 (369)400 4 286 0.68 7.1 42^(d) 26 (414) 400 4 313 0.64 6.0 43^(e) 19 (389)400 4 279 0.70 7.5 ^(a)Thermal treatment temperature in dynamicoxidizing atmosphere. ^(b)Thermal treatment by calcining in a staticoxidizing atmosphere, i.e. in an oven. ^(c)Thermal treatment in dynamicnitrogen atmosphere. ^(d)Thermal treatment in dynamic reducingatmosphere composed of a mixture of 5% by volume of hydrogen in N₂^(d)Thermal treatment in dynamic reducing atmosphere composed of amixture of 30% by volume of hydrogen in N_(2.)

What is claimed is:
 1. Nanomaterial of hydrogen titanate and/or mixedsodium and hydrogen titanate, having the formula HTiO₂ and/orNa_(x)H_(1-x)TiO₂, wherein x is from 0 to less than 1, the hydrogen andsodium atoms are in the interlayer regions of the orthorhombic structureand said nanomaterial has a nanotubular morphology constituted of piledstructural layers that are folded or rolled inwards into themselves, orformed by overlapped semitubes.
 2. The nanomaterial of claim 1, whereinsaid nanomaterial has an orthorhombic structure, which is isostructuralwith TiO_(2-x), where 0≦x≦1 and whose unit cell is described by at leastone of the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.
 3. Thenanomaterial of claim 1, wherein the cell parameters for the spacegroups of said orthorhombic crystalline lattice are, respectively: afrom 0.263 nm to 0.331 nm; b from 0.332 nm to 0.448 nm, and c from 0.635nm to 0.902 nm, for the space group 59 Pmmn, and a from 0.263 nm to0.331 nm; b from 0.332 nm to 0.448 nm, and c from 1.368 nm to 1.905, forthe space groups 63 Amma, 71 Immm, and 63 Bmmb; with α=β=90°.
 4. Thenanomaterial of claim 1, wherein said nanomaterial has a disorderedmesoporosity with an average pore diameter between 3 and 25 nm and aspecific area between 100 and 600 m²/g.
 5. The nanomaterial of claim 1,wherein said nanomaterial has a nanotubular morphology with dimensionsof 3 to 50 nm in diameter and lengths from 0.1 to 100 μm.
 6. Thenanomaterial of claim 1, wherein said nanomaterial has a nanotubularmorphology constituted by 1 to 50 structural layers with an inter-layerspacing in the range of 0.635 nm to 0.902 nm for the 59 Pmmn spacegroup, and from 0.684 nm to 0.953 for the 63 Amma, 71 Immm, and 63 Bmmbspace groups.
 7. The nanomaterial of claim 1, wherein said nanomaterialis hydrogen titanate and a mixed sodium and hydrogen titanate in whichthe hydrogen and sodium atoms are in the interlayer regions of theorthorhombic structure.
 8. The nanomaterial of claim 7, wherein saidnanomaterial has an orthorhombic structure, which is isostructural withTiO_(2-x) where 0≦x≦1 and whose unit cell is described by at least oneof the space groups 59 Pmmn, 63 Amma, 71 Immm or 63 Bmmb.
 9. Thenanomaterial of claim 1, wherein said nanomaterial has the X-raydiffraction pattern of FIG. 4 curve (b).
 10. The nanomaterial of claim1, wherein the nanomaterial has a nanotubular morphology with aninternal diameter between 3 and 10 nm and a specific surface areabetween 380 and 470 m²/g and an average pore diameter between 5 and 8nm.
 11. A method for the production of nanotubes of hydrogen titanateand/or mixed sodium and hydrogen titanate comprising contacting atitanium oxide of the anatase phase, said anatase phase titanium dioxidehaving a crystal size between 5 and 10 nm with sodium hydroxide to forma suspension, subjecting said suspension to a synthesis procedure withreflux in a closed system with continuous stifling between 100-500 rpmunder autogenous pressure at a temperature in the range of 80° to 120°C. to form a hydrogen titanate and/or a mixed sodium and hydrogentitanate with orthorhombic structure.
 12. Nanotubes of hydrogen titanateand/or mixed sodium and hydrogen titanate, having the formula HTiO₂and/or Na_(x)H_(1-x)TiO₂, wherein x is from 0 to less than 1 madeaccording to the process of claim 11.